Optical non-destructive evaluation (NDE) has been used in numerous materials characterization applications in the aerospace, semiconductor, and other industries. Conventional optical NDE methods are based on speckle interferometry, speckle photography, or double-exposure holography. Speckle-based methods take advantage of the speckle patterns produced by the scattering of monochromatic light, such as a laser light flux, from a rough surface. As a result, laser based optical speckle methods do not require specimen contact to assess specimen properties.
Speckle-based methods tend to be unsuited to measurement of many specimens of interest, frequently because of difficulties associated with either the application of a test stimulus or interrogation of the response to the stimulus. In addition, conventional methods frequently do not provide sufficient measurement resolution. While some optical NDE methods have been used to evaluate biological specimens in medical research applications, these methods are generally unsuitable for clinical applications. For example, clinical strain measurements of biological tissues or cell distortions are especially difficult. Accordingly, improved measurement methods and apparatus are needed.